This invention relates to an actuation device intended for a sewing means or sewing machine, having
a presser means for holding down the sewn material during stitching and material transport,
as the actuating element for the presser means at least one linear motor with a drive rod for controlling the presser force which is applied by the presser means to the material, which rod is connected to the presser means.
The presser means of a sewing machine (hereinafter the expression xe2x80x9csewing machinexe2x80x9d also comprises sewing means, this invention relating both to sewing means and also to sewing machines) is used to fix the position of the sewn material during needle puncture and to apply a pressing force to the material in the transport phase by which the transport means can push the sewn material onward.
In the past, in this connection efforts were made to produce the pressing force applied by the presser means using electromagnets. Since as the sewing speed becomes greater the presser force necessary for perfect interaction with the transport means is very high, attempts in this respect with electromagnets have failed among others due to the nonlinear behavior of electromagnets with respect to current flow and force in conjunction with the respective position.
One alternative approach is described in U.S. Pat. No. 4,214,540. According to this document a spring presses on the presser foot via a lever, the lever being supported in the middle. A linear motor engages the bearing point so that by shifting the bearing point the lever ratios change and thus the force on the presser foot is adjusted.
In this principle which is presented in U.S. Pat. No. 4,214,540 however it has been especially problematical that the high force of the presser foot is indirectly adjusted by a relatively weak linear motor. The technical implementation in this respect is not only complicated, but also costly since very high positioning accuracy of the linear drive is necessary.
U.S. Pat. No. 5,551,361 discloses a sewing machine in which the presser means is designed to apply to the sewn material a constant pressing force which is not influenced by the force acting on the feed dog. For this purpose the pressure bar is connected to a force sensor which is used to determine the current presser force. These measurement signals are converted into control signals for a linear motor which is attached to the sewing machine housing, the pressure bar moreover forming the drive rod of the linear motor.
Aside from the fact that in this presser means the control engineering complexity is considerable, there is another major disadvantage that the pressure bar is directly connected to the drive rod of the linear motor and the two rods form a common component. Since this rod additionally bears another coil or the movable parts of the linear motor, the mass to be moved is very great overall. To control the corresponding forces of mass inertia therefore the linear motor especially at high rpm of the sewing machine must apply very high forces so that the presser foot does not rise off the feed dog. The high forces to be applied in turn cause strong vibrations of the sewing machine.
Proceeding from the above described defects and deficiencies the object of this invention is to make a generic actuation means such that the force is transferred to the presser means in a direct, mechanically simple and still exact manner.
This object is achieved by an actuation means intended for a sewing means or sewing machine in which the drive rod with the presser means is connected via at least one elastic, low-mass coupling element and in which the presser means can be moved by the linear motor between the raised position and the lowered position.
Since the actuation device as the actuating element for the presser means has at least one linear motor with a drive rod for controlling the presser force which is applied by the presser means to the material, which rod is connected to the presser means, the force is transferred to the presser means in a direct, mechanically simple and still exact manner. Since the action of the force in the linear motor depends on the direction of current flow, the presser means is raised in one direction of current flow and pressed down in the other direction of current flow so that the presser means according to the teaching of this invention can be moved by the linear motor between a raised position and a lowered position.
For this reason the linear motor is suited as the direct actuating element for the presser means in an especially convincing manner; this differs from the approach which was common in the past, specifically producing the pressing force by pretensioning of a spring, in that the latter technically conventional approach to lifting the presser means requires an additional actuating element, for example an electromagnet. In contrast, the linear motor in this invention is used not only for producing the presser force, but is also used for lifting and lowering the presser means.
According to the teaching of the invention, on the other hand, the drive rod is connected to the presser means via at least one elastic, low-mass coupling element. In this way the drive rod of the linear motor is decoupled from the presser means so that the masses to be moved are kept small.
In this connection, keeping the moving masses small in a presser means is known. Thus for example DE-A-32 17 826 discloses a hollow pressure bar in which a correspondingly formed shaft of the presser foot is movably held. A spring which is supported for its part on an adjustable rod and which is located in the pressure bar presses on the shaft. The shaft of the presser foot is held by means of a holding plate in a somewhat larger slot of the hollow pressure bar, enabling limited vertical pushing motion of the presser foot relative to the hollow pressure bar. A second stronger spring exerts an adjustable pressure force on the hollow pressure bar via a guide part which is attached to the hollow pressure bar, the spring path of the pressure bar being limited by a collar which is attached to it and which is supported on a fixed guide bush.
This construction for reduction of moving masses is tailored to a presser means with two springs and due to measures for limitation of the spring path is unsuited to the presser means as claimed in the invention with only one elastic, freely movable coupling element. This construction could not therefore encourage one skilled in the art to develop the actuation device as claimed in the invention.